assertion and company

oh, i 'm sorry, i've given you a blank mail :P

i begin to say: Sorry for my bad english ^^


My problem is:
i have 2 tables,"editoriale" and "ColaboratoreFisso", in the first i have the foreign key of the second and in this
lasttable i have an attribute named MembroCR that can assume only boolean values... 

Now, i have to be sure that every row in "ColaboratoreFisso", that has the key also inside "editoriale", has the
attributeMembroCR=TRUE. 

So i thought to resolve this with an Assertion, but postgresql doesn't implement it... in what way i can solve my
matter?

This is my code (my first database, so don't blame me if i have writed too much mistakes :P)

CREATE TABLE editoriale (
    CodiceE serial PRIMARY KEY,
    Titolo varchar(100) NOT NULL,
    Testo text NOT NULL,
    MembroComitatoRedazione char(16) NOT NULL,
    FOREIGN KEY (MembroComitatoRedazione) REFERENCES collaboratoreFisso (CodiceFiscaleF)
           ON DELETE CASCADE ON UPDATE CASCADE);

     CREATE ASSERTION CONTROLLO_COMITATO
            CHECK (NOT EXISTS
            ( SELECT MembroComitatoRedazione
              FROM editoriale
              WHERE MembroComitatoRedazione IN
                (SELECT CodiceFiscaleF
                 FROM collaboratoreFisso
                 WHERE MembroCR = FALSE)));


if you can write me an alternative soulution^^



ANOTHER QUESTION:

I have some problem with my redundancies:

In what way i can implement them? With triggers? I have tried but my code doesn't work, i don't know how to use them :(


Thanks for your help! ^^